US3274067A - Fuel rod design - Google Patents

Description

Sept. 20, 1966 P. GREEBLER ET AL 3,274,067

FUEL ROD DESIGN Filed June '7, 1965 5 Sheets-Sheet 1 INVENTORS KENNETH M. HORST BY EUGENE E. OLICH ATTORNEY PA UL GREEBLER- BERTRAM WOLFE.

Sept. 20, 1966 P. GREEBLER ET AL FUEL ROD DES IGN Filed June 7, 1965 5 Sheets-Sheet 2 A MQ\ \Di mm mm ,9 mm

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INVENTORS PAUL GREEBLEF? KENNETH M HORST ATTORNEY United States Patent 3,274,067 FUEL ROD DESIGN Paul Greebler, Kenneth M. Horst, Eugene E. Olich, and

Bertram,Wolfe, all of San Jose, Calif assignors to the United States of America as represented by the United States Atomic Energy Commission Filed June 7, 1965, Ser. No. 462,145 3 Claims. (Cl. 176-68) The invention described herein was made in the course of, or under, contract No. AT(04-3)-540 with the United States Atomic Energy Commission.

This invention relates to fuel rods for nuclear reactors and in particular to fuel rods having segmented fuel portions arranged to thermally expand and contract with respect to each other in a compensatory manner to achieve a thermal expansion coefficient of reactivity approaching zero.

It is desirable, for the purpose of easily and accurately measuring the Doppler coefficient of a nuclear reactor core and isolating the phenomena of Doppler broadening from other neutronic phenomena of a nuclear reactor, to reduce the effect of change in reactivity due to axial thermal expansion of the fuel to a minimum or preferably zero. It is additionally desirable in a reactor where fast or high energy neutrons are utilized to maintain the fission reaction, to increase neutron economy at high power levels by reducing neutron leakage. Increased neutron leakage results when the outer surface area of an elongated cylindrical core increases due to axial thermal expansion of the fuel. The increase in volume and corresponding decrease in reactivity due to neutron leakage will thus mask out the reduction in reactivity due to Doppler broadening. The use of Doppler broadening for control of excursions in fast neutron reactors is described in detail in copending application of Paul Greebler, Serial Number 345,056, filed February 14, 1964 and is incorporated herein to the extent that it is pertinent to the description herein.

The present invention permits the effect of Doppler broadening to be measured directly and additionally permits its isolated use to reduce the effect of a rapid increase in reactivity without influence from other reactor core phenomena. The present invention accordingly greatly simplifies the control of the reactor using the aforesaid Doppler effect or in other instances in which the thermal expansion effects on reactivity complicates control.

In some reactor cores of the prior art, fissile fuel material is normally enclosed in individual containers and serially arranged in a fuel rod or fuel element primarily for the purpose of containing dangerous radioactive fission products. In other reactor cores of the prior art, the fissile fuel material is enclosed in an individual container or compacted to form a high density ceramic pellet within a cladding such as a fuel rod. In these prior art fuel rods, a space was sometimes provided at random or at regular intervals between the individual fissile fuel containers or dense ceramic pellets and the fuel cladding to allow for expansion of the fuel material. The spacing was provided for the primary and only purpose of preventing structural buckling of the fuel rod where both ends of the rod were held in a rigid or fixed relation to each other such as in a pressurized water reactor. No

effort was made, in the reactor cores of the prior art, to utilize in an effective manner thermal expansion of properly constructed fuel segments into properly disposed gap regions to reduce or modify the thermal expansion coefficient of reactivity.

In the method and device of the present invention, the fuel is divided into segments which are arranged to ex- 3,274,057 Patented Sept. 20, 1966 pand individually into predetermined gap areas and are of a specified length, specified in number and arranged in a specified manner within the core to achieve a zero or near zero thermal expansion coefiicient of reactivity. Incidental thereto, accommodation to linear thermal expansion is also achieved.

It is, therefore, an object of this invention to provide a fuel rod having an overall thermal expansion coefiicient of reactivity approaching zero.

It is a further object of this invention to provide a fuel rod whereby the Doppler broadening effect may be used as the primary means for controlling a reactor excursion or otherwise controlling the operating power level of a neutronic reactor.

It is another object of this invention to provide a fuel rod wherein the fuel is segmented and expands and contracts independent of the outer cladding.

Other and more particular objects of this invention will be manifest upon study of the following detailed description when taken together with the accompanying drawing, in which:

FIGURE 1 is a longitudinal section of a two-segment embodiment of the fuel element showing the parts in detail;

FIGURE 2 is a longitudinal section of a two-segment embodiment of the fuel element showing basic features as to segment length;

FIGURE 3 is a longitudinal section of a three-segment embodiment of the fuel element showing basic features as to segment length;

FIGURE 4 is a longitudinal section of a four-segment embodiment of the fuel element showing basic features as to segment length;

FIGURE 5 is a longitudinal section of a five-segment embodiment of the fuel element showing basic features as to segment length; and

FIGURE 6 is a graph illustrating the effect of varying the position of the inner void along the length of a twosegment fuel element.

The thermal expansion reactivity coefficient is defined herein as the change in reactivity for change in thermal expansion per unit length (e.g. centimeters along the longitudinal axis of the fissile fuel at temperatures close to normal or rated operating power. It will be appreciated that such expansion is not constant at higher or lower temperatures; however, the specific value of any chosen operating level does not modify the principle of operation and is not to be construed as controlling of the principle, concept or operations of this invention.

Referring to FIGURE 1, the fuel element of this invention comprises essentially a tubular cladding 11 encasing a fuel zone 12 between upper and lower end neu tron reflector plugs 13 and 14, respectively, with fuel zone 12 including solid fissile fuel material divided into at least two and no more than five segments slidably mounted therein. Upper and lower fuel segments with lengths selected in accord with principles discussed more fully hereinafter are retained in compression between spring means and a fixed stop such that it is fixed at its lower end and allowed to expand against spring means, e.g., 28 or 31 into a void 16 (for lower segment 23) or 32 for upper segment 3-0 at its upper end. Such segments can be provided as an appropriate number of separately clad subsegments 17 serially arranged along the length of the fuel element, in groups, in loose fitting relation inside cladding 11 such that cladding 11 will not be carried along by axial expansion of subsegments 17. The lengths and number of fuel segments are mounted and particularly arranged as hereinafter described to reduce the thermal expansion coefiicient of reactivity to approach zero. In essence one of said segments 30 is mounted within said cladding with the lower end fixed in relation to said cladding and the upper end expansible into a void 32 at the upper end of said cladding. A second segment is fixed to said cladding below the midplane with the upper end extending across the midplane and expansible into a void 17 effectively between the top thereof and the lower end of segment 30.

The foregoing basic arrangement is provided with appropriate modifications, described hereinafter, to include from 2 to 5 fuel segments. A typical embodiment for a two-segment fuel element is illustrated in FIGURE 2 illustrating the disposition and the length of segment necessary for operation in accord with this invention. Basically the construction details for the two segment fuel element will also be applicable to fuel elements with more than two segments.

For mounting in a reactor core vessel (not shown) referring to FIGURE 1, a bottom support pin 21 is provided at the lower end of cladding 11 and is .aflixed as by welding or the like thereto. Bottom support pin 21 is arranged to be received and supported in a recess or hole in a grid plate (not shown) in a reactor containment vessel (not shown). Above and in immediate contact with the bottom support pin 21 there is disposed within said cladding 11 a stop 14 serving as a lower neutron reflector which is fabricated from nickel, carbon or like neutron reflective material. Adjacent above lower neutron reflector 14 is lower segment insulator pellet 22 fabricated from any high temperature thermal insulation material, e.g., aluminum oxide, magnesia, sapphire or the like. In contact with said insulation pellet 22 is, for the two segment fuel element embodiment, the lower end surface of a relatively elongated lower fuel segment 23 comprised of a grouped plurality of serially arranged subsegments 17. Each subsegment is an individually clad compact containing fissile fuel, e.g., U PuO mixtures thereof for the like. Subsegment cladding 25 may comprise any one or more of the usual nuclear reactor grade stainless steels or an alloy of zirconium or other low neutron absorbing metal. The outside diameter of the subsegments is arranged to be less than the inside diameter of fuel element cladding 11 at operating temperatures to permit unobstructed axial movement of segment 23 within cladding 11 with relation to insulator 22 and stop 14. In immediate contact with the topmost subsegment of lower fuel segment 23 is lower segment upper insulator pellet 26 of diameter at operating temperature equal to the diameter of subsegment 17. Above said insulator 26 is void 16 and above void 16 is plug 27 aflixed by welding or the like to the upper end of the lower portion 29 of fuel element cladding 11. In void 16 in compression against both plug 27 and insulator pellet 26 is spring 28 which holds the serially arranged subsegments 17 in place against insulator pellet 22 and lower neutron reflector 14. Thus lower fuel segment 23 can only expand upwardly against spring 25 into void 16. Moreover, lower cladding portion 29 can independently expand upwardly carrying plug 27 therewith.

In a like manner, upper fuel segment 30 is arranged with the lower end of upper portion 33 of fuel element cladding 11 surrounding said segment 30 which portion 33 is aflixed at its lower end as by welding or the like to plug 27. The upper end of upper fuel segment 30 compresses top spring 31 in top void 32 against upper neutron reflector stop 13 fabricated of material similar to lower neutron reflector 14, which reflector 13 in turn is in immediate contact with top coupling 34. Top coupling 34 is afiixed as by welding or the like to fuel element cladding 11. Top coupling 34 is also provided with recess 35 into which male plug end 37 of extension rod 38 fits and is attached as by screws, rivets, pins or the like. Extension rod 38 is used for both inserting and removing the fuel element into and from the bundle of like fuel elements comprising the core of the reactor and additionally can act as a radiation shield over the top of the core. Upper and lower ferrules 40 and 41 are arranged to abut like ferrules of adjacent fuel elements with slots 42 of ferrules 40 and 41 arranged to prevent neutron streaming along the rod causing activation of the reactor vessel head (not shown). Extension rod head 39 is provided at the upper end of extension rod 38 .adapted to receive a grappling means (not shown) for lifting the extension rod-fuel element unit out of the reactor.

Of particular note is the location of void 16 in the two-segment fuel element. Whereas the three-, four-, and five-segment fuel elements have segments of equal lengths, it has been found that for the two-segment fuel element, the void 16 at the top of lower segment 23, must be above transverse mid-plane 50 of the fuel element with the upper end of segment 23 movable thereinto with the lower end fixed below said midplane. FIGURE 6 is a graph showing the percentage reduction in thermal expansion .coeflicient of reactivity as a function of location of void 16 within the core along the length of the twosegment fuel element. Curve 45 illustrates the percentage reduction in the thermal expansion coeflicient of reactivity for isothermal or steady state heating. Curve 46 illustrates the same effect for a fast transient heating as would occur during a sudden increase in reactivity. It will be noted that point 47, where the thermal expansion coeflicient of reactivity is at a minimum, occurs above midpoint 48 which corresponds to transverse midplane 50 of the fuel element, of FIGURE 2. For a two-segment fuel element having a total fissile fuel segment length of 85.94 cm., point 47 is approximately 10 cm. above tranverse midplane 50 (point 48 on curves 45 and 46 of FIGURE 6) of the fuel element.

It has been found that for the three-, four-, and five-segment fuel elements, the length of segments may be made equal to achieve the desired results, a circumstance which fortuitously simplifies fuel fabrication. It has also been found that the use of more than five segments will introduce too great a volume of neutron absorbing material into the reactor core for the benefit gained in further reducing the thermal expansion coefficient of reactivity.

For the three-, four-, and five-segment fuel element as illustrated in FIGURES 3, 4 and 5 respectively, theconfigurations for upper reflector 13 and above as well as lower reflector 14 and below is identical to the twosegment fuel element of FIGURES 1 and 2. The threesegment fuel element of FIGURE 3 comprises a top void 32 immediately below upper reflector 13, a first upper insulator pellet 51 held in compression against serially arranged subsegments in upper fuel segment 30 by spring 28 located in top void 32. The lower end of upper fuel segment 30 rests against first lower insulator pellet 52 which in turn rests against plug 53 fabricated from stainless steel or the like. Cladding 11 of the fuel element is aflixed as by welding or the like to said first plug 53. Immediately below first plug 53 arranged in a manner similar to top void 32, first upper insulator pellet 51, upper segment 30, first lower insulator pellet 52, and first plug 53 is second void 55, second upper insulator pellet 56, second segment 57, second lower insulator pellet 58 and second plug 59. Second spring 60 is arranged in second void 55 to maintain second segment 57 in compression. Immediately below second plug 59 in a like manner as above, is arranged third void 63 containing a third spring 64, third upper insulator pellet 65, third segment 66 and third lower insulator pellet 67 in compression against lower neutron reflector 14.

The four-segment fuel element is similar to the three segment element however with third plug 69, fourth void 70 containing a fourth spring 71, fourth upper insulator pellet 72, fourth segment 73 and a fourth lower insulator pellet 74 in compression against lower neutron reflector 14 all serially arranged below third insulator pellet 67.

In a like manner, for the five-segment fuel element, fourth plug 75, fifth void 76, fifth spring 77, fifth upper insulator 78, fifth segment 79 and fifth lower insulator pellet 80 in compression against lower neutron the bottom segment expands upward into its void, the top segment will also expand upward into its void but will also, as a unit, be moved upward away from the bottom segment due to the axial thermal expansion of reflector 14 are all serially arranged below fourth insu- 5 cladding 11. During this period the fuel element of this lator pellet 74. invention will not adequately operate in the manner To more aptly demonstrate the effect of segmenting above described to compensate for the change in the the fuel as described above, Table I is arranged to show thermal expansion coefiicient of reactivity. It is predata regarding examples of two-, three-, four-, and fiveferred that the fuel element of this invention operate segment fuel elements of this invention compared with when the reactor reaches operating power such that the a fuel element identical in all respects to the above with rate of flow of coolant may be varied to maintain a the exception that the fuel is not segmented. In all five constant cladding temperature while the temperature of examples, the amount of fissile fuel contained in each the fuel and resulting expansion of the fuel may vary in fuel element is the same. The numeral preceding the response to the reactivity level. Also, in the event of fuel element part is the reference character for a corresa sudden increase in reactivity resulting in a sudden inponding dimension line shown in FIGURES 2, 3, 4, and crease in fuel temperature, a finite time period will elapse 5 as applicable to the two-, three-, four-, or five-segment before the fuel element cladding increases in temperature. fuel element respectively. Although a fuel element hav- This time period is usually of the order of milliseconds. ing no fuel segmentation is listed in Table I, no drawing In either case, axial thermal expansion of the cladding has been included of the unsegmented fuel element since will not affect the operation of the fuel element of this it is not a part of nor represents an embodiment of this invention. invention and is used only for comparison. It is be- Calculations for determining the thermal expansion lieved a person skilled in the art will readily understand its coeflicient of reactivity can be performed accordingly for construction. Table II includes data pertinent to a typical a one-dimensional system using diffusion theory by digital fast neutron reactor which would beneficially use the 2 computer means through iterative computations. Bafuel rods of this invention to form a reactor core. sically neutron leakage plus neutron absorption is bal- When a nuclear reactor core, comprising a plurality anced against the source of neutrons from fission reacof segmented fuel elements of this invention, is placed in tions by repeated calculations by the digital computer. operation and raised from room temperature to operating Within the accuracy of these calculations, the number of power it is obvious that both cladding 11 as well as the segments required for the thermal expansion coefiicient fuel segments will expand axially due to their inherent of reactivity to reach zero is found to not exceed five linear thermal expansion properties which, for the and increases in the number of segments thereafter concladding of stainless steel is 9.8 l0 /F., and for the tinues at a zero level. No further improvement is thereby fuel is 5.4 l0 /F. Thus from startup to operating gained and an increasing penalty in the increase of extemperature the segments will expand concurrently with traneous materials results. A four or five segment fuel a relative expanding movement between segments. As element is accordingly near optimum.

TABLE I Fuel Element Configuration No Segmentation Two-Segment Three-Segment Four-Segment Five-Segment (No drawing) (Figure 2) (Figure 3) (Figure 4) (Figure 5) Dimension Reference Item Character Cm. Om. Cm Cm. Extension Rod 38 (Neutron Shield) and 38.10 38.10 38. 10 38. 10 38. 10

Connectors. Upper Reflector l3 10.19 10. 19 10.19 10.19 10. 19 Top Void 32ml. 2. 35 1. 94 1.50 0.88 First Upper Insulator Pellet 51. 0.95 0. 95 0.95 0.95 Upper Fuel Segment 30 32.96 28.65 21. 47 17.19 First Lower Insulator Pellet 52.. 0.95 0.95 0.95 0.95 First Plug 27 (Fig. 2) or 53 (Figs. 4, 1.59 1. 59 1. 59 1. 59

an 5 Seeogd )Void 16 (Fig. 2) or 55 (Figs. 3, 4 4.19 2. 78 1.85 1.61

an 5. Second Upper Insulator Pellet 26 (Fig. 0. 95 0.95 0.95 0.95

2) or 56 (Figs. 3, 4 and 5). Second Fuel Segment 23 (Fig. 2) or 57 52.98 28.65 21.48 17.19

(Figs. 3, 4 and Second Lower Insulator Pellet 22 (Fig. 0.95 0.95 0.95 0.95

2) or 58 (Figs. 3, 4 and 5). Second Plug 59 1. 59 1. 59 1. 59 Third Void 63 1. 83 1.80 1. 67 Third Upper Insulator Pellet 65 0.95 0.95 0. 95 Third Fuel Segment 66 28. 65 21. 48 17. 19 Third Lower Insulator Pellet 67 0.95 0.95 0.95 Third Plug 69 1. 59 1. 59 Fourth Void 70 1. 40 1. 61 Fourth Upper Insulator Pellet 72 0.95 0.95 Fourth Fuel Segment 73. 21.48 17. 19 Fourth Lower Insulator Pellet 74 0.95 0. 95 Fourth Plug 75- 1. 59 Fifth Void 76 0.77 Fifth Upper Insulator Pellet 78-- 0.95 Fifth Fuel Segment 79 17. 19 Fifth Lower Insulator Pellet 80 0. Lower Reflector 14.-. 10.19 10.19 10.19 10.19 10.19 Bottom Support Pin late, 43.18 43.18 43.18 43.18 43.18

Sodium and Gamma Shield. Reactor Core Diameter- 86. 07 86.07 86. 07 86. 07 86.07 Fuel Element Diameter 2. 54 2. 54 2. 54 2. 54 2. 54 Fuel Pellet Diameter.. 2. 24 2. 24 2. 24 2. 24 2. 24 Fuel Temperature, F 4, 500 4, 500 4, 500 4, 500 4, 500 Cladding Temperature, T 800 800 800 800 800 O fi/eg/ tiug Power (Total for Reactor), 20 20 2O 20 20 Thermal Expansion Ooefiicient of Re- 0. 0038 0. 0011 0. 0001 zero +0. 0006 activity.

Referring to Table I, it can be seen that the absolute value of the thermal expansion coefiicient of reactivity has been substantially reduced by substituting a segmented fuel element for the non-segmented fuel element of conventional design. It can be seen that to increase segmentation of the fuel beyond five segments would result in the coefficient remaining near Zero or going slightly positive thus gaining no additional benefit from segmentation. On the contrary, additional segmentation would tend to be disadvantageous by introducing more neutron absorbing material in the core thus reducing neutron economy and tending to require more fissile fuel for the same power output. Thus segmentation of the fuel and the provision of voids as above described causes the thermal expansion coefficient to approach, and in the case of the four-segmented fuel element, reach zero thus permitting the thermal expansion of the fuel to not alfect reactivity of the reactor.

Although the foregoing embodiment has been described in detail, there are obviously many other embodiments and variations in configuration which can be made by a person skilled in the art without departing from the spirit, scope or principle of this invention. Therefore, this invention is not to be limited except in accordance with the scope of the appended claims.

TABLE II Reactor type Fast neutron,

heterogeneous. Fuel:

Mixed PuO -UO pelletized,

total enrichment 0.168 atom percent. Moderator materials (volume fraction):

BeO 10.02 percent. Stainless steel 23.77 percent. Sodium 20.88 percent. Mean neutron energy 120 kev. (-max.

energy 2.5 mev.). Power output 20 mw. Reflector control, nickel rods:

Total number 10. Max. reactivity per rod 28. Total stroke 35 inches. Heat transfer system:

Primary, sodium. Secondary, sodium. Tertiary, air. Containment, main vessel (type I.D. at fissile fuel section 40 inches. ID. at upper section 57 inches. Wall thickness 0.375O.750 in. Design pressure 5O p.s.i.g. Core:

Diameter 33.8 inches. Fissile fuel length 33.8 inches. Neutron flux (peak 1 mev.) 1 10 neut./

em. sec. Fuel rod:

Overall length 120 inches. CD. at fissile fuel section 1.0 inch. Total No. in core 618. Cladding Type 316 stainless steel.

[Hexagonal array with central moderator rod] Moderator rod:

BeO total No. in core 108. Shroud:

Hexagonal, diameter across flats 3.066 in. Length 106 in. Wall thickness 0.060 in. Material Type 316 stainless steel.

8 Extension rod:

Length neutron shield 48.0 in. Length above neutron shield 26 in. Maximum O.D. 0.88 in. Material Type 316 stainless steel and B C. Clad thickness 0.040 in. Coolant:

Inlet temperature 700 F. Outlet temperature 820 F. Flow rate at power 4360 g.p.m. Velocity in core 8.6 ft./sec. average. 13.0 ft./sec. max. Heat transfer coefiicient 20,000 B.t.u./hr.

ft. F. Fuel loading:

U 1598 kg. Pu 26 kg. Pu 296 kg.

Total 1920 kg.

Fuel composition:

PuO UO 45.4 volume percent. BeO 11.2 volume percent. Coolant (sodium) 19.2 volume percent. Structure (steel) 23.8 volume percent. Physics data:

Fraction of fissions below 9 kev 0.22. Prompt neutron lifetime 036x10- sec. Total neutron flux Core center 6.4x 10 n/crn.

sec. Average 3.4 10 n/cm.

sec. Gamma and neutron heating- Prompt fission 'y 7.8 mev./fission. Fission product 'y 7.2 mev./fission. (n, 7) Reaction 8.2 mev./fission. Inelastic scattering 2.4 mev./fission. Elastic scattering 3.6 mev./fission.

Total 29.2 mev./fission.

What is claimed is:

1. In a fuel element for obtaining a thermal expansion coefiicient of reactivity approaching zero and facilitating control of the rapidly rising reactivity of a nuclear reactor through the use of the Doppler broadening effect, the combination comprising an elongated tubular cladding having upper and lower closed ends defining a fissile fuel zone within said cladding, a plurality of from two to five serially arranged fuel segments in said zone, each segment having one end immovably fixed relative to said cladding within said fuel zone and with the free end of at least one of said segments together with a proximate fixed end of a second segment and the free end of the uppermost segment together with one end of said fuel zone defining void spaces of a length greater than the maximum thermal expansion length of said fuel segment at any temperature below the melting point of said fuel, resilient restoring means disposed within said void spaces, each restoring means affixed proximate one end thereof relative to said cladding, with the other end bearing against the free end surface of fuel segment to maintain the fixed end in position with respect to said cladding to permit each of said segments to thermally expand and contract in unison in the same direction.

2. A fuel element as defined in claim 1, wherein said segments are three to five fuel segments of equal length, and which are serially arranged within said fuel zone.

3. A fuel element as defined in claim 1, wherein a first fuel segment is affixed relative to said cladding proximate the lower end of said fuel zone extending into a region approximately 67% of the length of said fuel zone from the lower end thereof, a second fuel segment is affixed relative to said cladding above the upper portion of said region 67% of the length of said zone and extending into the upper portion of said fuel zone, With the upper end of said first fuel segment together with the lower end of said second fuel segment defining a first void space between said segments and the upper end of said second segment and the upper end of said fuel zone defining a second void space, said void spaces having a length greater than the maximum thermal expansion length of said fuel segment at any temperature below the melting point of the fuel.

References Cited by the Examiner UNITED STATES PATENTS 2,852,460 9/1958 Abbott et a1 176-82 X 2,969,674 1/1961 Ogle 176-76 X 3,053,743 9/1962 Cain 17676 X 3,085,059 4/1963 Burnham 17673 3,105,030 9/1963 McGeary et al 17673 3,184,392 5/1965 Blake 17673 X References Cited by the Applicant UNITED STATES PATENTS 2,994,656 8/ 1961 Zumwalt.

3,010,889 11/1961 Fortescue et a1.

3,039,944 6/ 1962 Zumwalt.

3,059,830 5/1963 McGeary et al.

BENJAMIN R. PADGETT, Acting Primary Examiner.

M. J. SCOLNICK, Assistant Examiner.

Claims (1)

1. IN A FUEL ELEMENT FOR OBTAINING A THERMAL EXPANSION COEFFICIENT OF REACTIVITY APPROACHING ZERO AND FACILITATING CONTROL OF THE RAPIDLY RESING REACTIVITY OF A NUCLEAR REACTOR THROUGH THE USE OF THE DOPPLER BROADENING EFFECT, THE COMBINATION COMPRISING AN ELONGATED TUBULAR CLADDING HAVING UPPER AND LOWER CLOSED ENDS DEFINING A FISSILE FUEL ZONE WITHIN SAID CLADDING, A PLURALITY OF FROM TWO TO FIVE SERIALLY ARRANGED FUEL SEGMENTS IN SAID ZONE, EACH SEGMENT HAVING ONE END IMMOVABLY FIXED RELATIVE TO SAID CLADDING WITHIN SAID FUEL ZONE AND WITH THE FREE END OF AT LEAST ONE OF SAID SEGMENTS TOGETHER WITH A PROXIMATE FIXED END OF A SECOND SEGMENT AND THE FREE END OF THE UPPERMOST SEGMENT TOGETHER WITH ONE END OF SAID FUEL ZONE DEFINING VOID SPACES OF A LENGTH GREATER THAN THE MAXIMUM THERMAL

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